Reduction and removal of process oxides on stainless steel

ABSTRACT

Oxides formed during annealing of stainless steel strip are removed with abrasive brushes, in lieu of acid or molten salt based pickling. In some embodiments, the stainless steel strip is treated with a rare earth element or a related transition metal before annealing, and then brushed after annealing to remove any oxides. The selection of brushes can impart a finished appearance to conventionally polished stainless steel.

PRIORITY

This application claims priority to U.S. Provisional Application Ser. No. 62/808,399, filed Feb. 21, 2019, entitled “Reduction and Removal of Process Oxides on Stainless Steel,” the disclosure of which is incorporated by reference herein.

BACKGROUND

The present application relates to mechanical removal of oxides formed during the processing of stainless steels.

During typical stainless steel strip processing, oxides are generated during high temperature anneal cycles used to soften the steel after cold rolling. Typically these oxides are removed prior to subsequent processing by employing conventional acid-based pickling techniques. Such descaling processes employ exposing the annealed stainless steel strip to a molten salt bath and/or one or more acids in an acid bath. Descaling is slow, costly, and can be environmentally challenging.

SUMMARY

In the present embodiments, it has been determined that the oxides formed during annealing can be removed with abrasive brushes. In some embodiments, the stainless steel strip is treated with a rare earth element or a related transition metal before annealing, and then brushed after annealing to remove any oxides. The brushing operation imparts a surface texture to the material that may eliminate the need for more conventional polishing and finishing practices.

DESCRIPTION OF DRAWINGS

FIG. 1 depicts the GDS depth profile for a 436L stainless steel sample after final anneal with no brushing.

FIG. 2 depicts the GDS depth profile for a 436L stainless steel sample after final anneal with one pass brushing.

FIG. 3 depicts the GDS depth profile for a 436L stainless steel sample after final anneal with two pass brushing.

FIG. 4 depicts the GDS depth profile for a 436L stainless steel sample after final anneal with three pass brushing.

FIG. 5 depicts the GDS depth profile for a 436L stainless steel sample after final anneal without any rare earth metal or rare metal treatment and no brushing.

FIG. 6 depicts the GDS depth profile for a 436L stainless steel sample after final anneal with Minimox treatment but no brushing.

FIG. 7 depicts an SEM image of a 436L stainless steel surface after final anneal/pickling showing grain pattern.

FIG. 8 depicts Chromeshield® 22 stainless steel cold-rolled samples after annealing, with the Minimox dipped samples on the right.

FIGS. 9a and 9b depict the GDS depth profiles for untreated and treated panels.

FIG. 10 depicts the annealed Chromeshield 22 stainless steel samples after Scotch-Brite brushing.

FIG. 11 depicts the annealed Chromeshield 22 stainless steel samples after bristle type brushing.

DETAILED DESCRIPTION

Annealing and pickling (“AP”) practices are commonly used in stainless steel processing. Annealing is necessary to soften the steel after the work hardening induced during rolling. Annealing is usually conducted in air-based atmospheres at temperatures of 1700-2000° F. for several minutes. The resulting oxides that are formed are then removed by a pickling step. The pickling reactions are typically the rate controlling step during AP. Most stainless steel is sold in this “fully annealed” condition with the annealing oxide removed. If the pickling step can be eliminated, not only are the acids and molten salts used in descaling no longer be required, but the line speeds may be able to be increased.

In some embodiments, mechanical brushing can be used to remove the oxides formed during annealing. Typically, the oxides formed after annealing are relatively thin (<1 micron) and modern brushing machines have been found to be capable of removing this surface layer.

In other embodiments, the effectiveness of the brushing may be enhanced if the stainless steel strip is treated with a rare earth metal or a related transition metal before annealing. This surface treatment affects the growth of oxides on stainless steels, decreasing their thickness and altering the surface chemistry to make them easier to remove.

The manufacturing of stainless steel strips is well-known. The present oxide removal processes (also known as descaling) can be implemented in a standard stainless steel manufacturing process, replacing only the acid-based pickling and/or molten salt portion of the process.

Brushing machines, and the brushes used with them, are also well-known. Brushing is often used to impart a desired surface finish to a clean (or bright) stainless steel strip, i.e., to a strip from which any oxides have been previously removed.

In certain embodiments of the present process, such brushing machines are used to brush oxide-bearing stainless steel strip, thereby removing the oxides from the surface of the stainless steel strip. In one embodiment, the brushing machines use Scotch-Brite™ nonwoven type rolls that contain a loose web of nylon or other fibers impregnated with alumina or silicon carbide type particles. Typical roll types would include the 3M CB Cleaning rolls and the CS-CB Clean and Strip rolls. Scotch-Brite is an abrasive material sold by the 3M Company, St. Paul, Minn.

The rare earth element or related transition metal can be applied to stainless steel strip in a liquid form, such as an aqueous suspension or an aqueous solution comprising metal salts. The rare earth element can include yttrium, cerium, lanthanum, and their associated oxides or nitrates. And the related transition metal can include zirconium and its associated oxides.

The liquid containing rare earth metal or related transition metal can be applied to a stainless steel surface prior to annealing by dipping, painting, and spraying and then dried.

In some embodiments, the rare earth element can be provided in a suspension. For example, Minimox® liquid, available from Materials Interface, Inc. of Sussex, Wis., comprises nano-particles of rare earth oxide in suspension. Such materials are also described in U.S. Pat. No. 8,568,538, the disclosure of which is incorporated by reference herein. It is believed that Minimox contains up to about 1% by weight yttrium as Y₂O₃ nanoparticles that are suspended in an aqueous medium.

An embodiment of the present invention comprises an aqueous solution of rare earth metal salt. The salt can be a nitrate or an acetate, for example. The rare earth metal salt can include one or more of cerium (Ce), lanthanum (La), and yttrium (Y). Chlorides can permit corrosion and carbonates may not soluble, thus, in the present application rare earth metal salts do not include carbonates or chlorides.

Rare earth metal salts are well known in the industry and commercially available. It is not necessary that the nitrate or acetate be of any particular grain size, and particularly there is no need for the salts, or the resulting rare earth metal oxides, to be limited to nanoparticles, which are considered to be particles with dimensions in the range of 1 to 100 nm.

The solution comprises rare earth metal nitrate or acetate dissolved in water, preferably deionized water. The concentration of the rare earth metal nitrate or acetate in the aqueous solution can extend to the limits of solubility of the particular salt. In certain embodiments, a solution can have a concentration equal to about 1 to about 10 g of rare earth metal nitrate or acetate to about 200 g of total aqueous solution. In other embodiments, a solution can have a concentration equal to about 1 to about 20 g of rare earth metal nitrate or acetate to about 200 g of total aqueous solution.

In some embodiments, the amount of the rare earth metal or related transition metal that is applied to the stainless steel surface has a density of about 300 to 3000 μg/m², or in some embodiments a density of about 500-8000 μg/m² or in other embodiments 5000-8000 μg/m².

To improve wetting of the stainless steel, a surfactant may be added to the aqueous solution or suspension. Surfactants are added to a concentration of about 0.1% to 5% by weight of solution, and in some embodiments, in a concentration of about 0.1% to 0.5% by weight of solution. Any surfactant known to enhance wetting of an aqueous solution or suspension onto a stainless steel surface can be used. The surfactant may comprise a detergent, such as dishwashing detergent.

The resulting aqueous solution or suspension can be applied to one or both surfaces—of a stainless steel strip (or to any other stainless steel product) by any method known to evenly apply liquids to a surface, including brushing, sponging, roll coating, spraying, and dipping. It is then dried using any method currently used to dry paints and other liquids on moving strip or webs such as forced air, infrared heating, convection ovens, etc.

It is believed the present invention benefits all types of stainless steel. The stainless steels that benefit from the present invention include ferritic, austenitic, and martensitic stainless steels.

To use the methods of the present invention, stainless steel strip is manufactured using conventional, well-known practices, except as follows. After annealing, rather than pickling/molten salt descaling, the stainless steel strip is brushed on one or both sides at least once. If rare earth metals or related transition metal solutions or suspensions are to be applied to the strip, they are applied to one or both sides of the stainless steel strip prior to annealing. If the stainless steel manufacturing process is a continuous process, the solution or suspension is applied in-line by methods known in the art, and the brushing is likewise done in-line. Alternatively, either or both processes can be performed as batch operations, or off-line, as well.

Example 1

Two 436L stainless coils provided by AK Steel Corporation, West Chester, Ohio, were each partially coated with Minimox nano-yttrium oxide rare earth based suspension and then annealed. In each case, along the length of the strip, approximately one-half was coated and one-half was not coated. Annealing was conducted in air with 3% excess oxygen at approximately 1950 F/1066 C. Line speed was 45 ft/min and time in the furnace was about 3.5 min.

After annealing, there was noted only a slight color difference on the surface of the strip between the coated portion of each coil and the uncoated portion. However, there was a more noticeable difference in the appearance on the sidewalls of each coil.

The scale on either the treated or untreated portion of the strip was easily removed by abrading the steel strip with an ultra-smooth, #400 grit alumina or silicon carbide abrasive paper.

Example 2

The stainless steel coils of Example 1 were brushed using two conventional brush stands having 3M Scotch-Brite nonwoven brushes typically used to remove red rust from carbon steel. There were two brush types. There were two brushes in tandem that contacted the top of the strip (“Maroon” medium aggressive Type CB cleaning brushes with aluminum oxide abrasive media particles) and two that contacted the bottom (“Black” heavy duty Type CS-CB Clean and Strip brushes with more aggressive silicon carbide abrasive media particles). Note that the brushes were not new but recently dressed and balanced. Brush speed was 250-300 surface ft/min and line speed was 45 ft/min. Work pressure was in the range of 0.1 to 0.5 horsepower per inch of working width.

After one pass through the brushing station, removal of the oxide on the top side was less than the removal on the bottom side due to the use of the more aggressive brushes on the bottom. The top was “dark” in appearance and the bottom “bright” indicating more residual oxide on the top and little remaining oxide on the bottom. There was also a noticeable linear pattern on the strip due to contact with the rolls. No visual differences were detected between the Minimox-treated and non-treated portions of the strip.

The strip was then passed through the line a second time. Removal was marginally more noticeable on the top side but the “dark” appearance was still present. Oxide removal was visually complete on the bottom side and the texture was smoother than after one pass. Again no visual difference was detected between the Minimox treated and non-Minimox treated portions.

The strip was then reversed so that the previous bottom side was now subjected to the less aggressive top side roll brushes and the previous top side with the more aggressive roll brushes. At this point the new top surface not only had a “bright” appearance but also had a smoother texture similar to standard #4 Polish stainless. #4 Polish is the preferred short line grit pattern directional finish for stainless steel that is typically used for appliance and food preparation surfaces and equipment. #4 Polish is a standard surface finish created by polishing with a 150 grit emery polishing belts as described in the Stainless Steel Comparator published by AK Steel Corporation, West Chester, Ohio. The bottom surface was also bright after this third pass. Therefore brushing operation can replace the expensive separate stainless steel process step of surface polishing to obtain the desired appearance on the finished product.

The visual appearance observations of the strip was supplemented by Glow Discharge Spectroscopy (GDS) measurement to verify removal of the oxide (FIGS. 1-4). The oxygen peak (0) moves closer to the surface (0 Depth location) as the number of brushing passes increases.

Note that in production, it may not be economical to run material through the line multiple times. Instead the number of brushes that would be used on a given process line would increase. For instance in the current study, two brushes per side were used. If two passes are needed to remove the oxide, then the production equivalent would be four brushes per side. If three passes are needed on a two brush line, then six brushes per side would be used.

The effect of the Minimox rare earth treatment was to reduce the thickness of the oxide that was grown during annealing. Thinner oxides require less removal and therefore less brushing. A comparison of the GDS results for material from the same coil annealed with and without oxide is shown in FIGS. 5 and 6. A measure of the relative thickness of the oxide is where the iron (Fe2) curve reaches the 50% Analyte Weight Percent point. In the case, the value of approximately 0.25 micron for the no treatment case vs 0.175 micron for the Minimox treated material would suggest a 30% reduction in oxide thickness. This would translate into a 30% faster process speed or 30% better life on a given set of rolls before dressing is needed. Note that 30% less removal would occur indicating less waste disposal.

Two additional advantages may be associated with brushing versus molten salt/pickling after final anneal. First, the brushing operation leaves a surface pattern from the abrasive action of the brushes. The final brushing type and operation can be selected to impart a preferred finish appearance to the material. The surface of molten salt/acid pickled material does not have a discernable pattern when viewed

Second, on a microscopic level, material that has been pickled has a pattern that is associated with the preferred etching of grain boundaries (FIG. 7). Although not visible, this pattern can result in poorer corrosion performance for the material. Since chemicals are not used in brushing, there is no etching of grain boundaries.

Example 3

Cold rolled samples of Chromeshield® 22 stainless steel strip provided by AK Steel Corporation, West Chester, Ohio, were annealed in air at 1875° F. (1024° C.) for 3.5 minutes to simulate strip anneal after cold rolling. The samples were alkaline cleaned prior to annealing and two were subsequently dip treated with Minimox 721 yttria nano particle 1% aqueous suspension for 5 seconds at room temperature and allowed to air dry. Visual differences between the treated and untreated sample were apparent after annealing. See FIG. 8.

The surfaces were examined via a depth profile using glow discharge spectroscopy (GDS). Results are shown for the untreated panels (See FIG. 9a ) and the treated panels (See FIG. 9b ). In both cases, the oxide region appeared to be less than 0.5 micron thick. Profiles for the treated samples may have been affected by non-uniform application of the Minimox suspension.

Samples were then brushed on one side with a Scotch-Brite roll using a laboratory brush unit with alumina particle impregnated rolls. Inspection indicated that the oxide was essentially removed after approximately four passes through the brush.

The two samples were also brushed on the opposite side using a silicon carbon impregnated bristle brush. In this case, removal of the oxide was not as complete and the surface was less uniform with visible lines.

The material brushed with Scotch-Brite rolls is shown in FIG. 10. The material brushed with the bristle brush is shown in FIG. 11.

Example 4

A 436L stainless strip is processed as follows at 45 ft/min after 3.5 minute anneal in a 3% excess oxygen air oven at 1950° F.:

The anneal oxide is removed from the stainless surface by subjecting the strip to between 2-4 3M Scotch-Brite silicon carbide impregnated brushes

A smoother surface is obtained by subsequently passing the material through 1-2 less aggressive 3M Scotch-Brite alumina impregnated brushes.

First brushing with an aggressive brush followed by a less aggressive brush results in a more uniform surface texture than using only aggressive brushes.

Higher line speeds require more brushing stations and/or more aggressive brushes. 

What is claimed is:
 1. A process for removing oxides from annealed stainless steel comprising the steps of a. Providing a stainless steel strip having at least one surface, b. Annealing said stainless steel strip, c. Removing oxides formed during said annealing step by brushing said at least one surface.
 2. The process of claim 1 further comprising the step of applying a rare earth metal or related transition metal to said at least one surface before the annealing step.
 3. The process of claim 1 further comprising the step of employing a finishing roll after the brushing operation that imparts an appearance similar to polished stainless steel. 